1. Technical Field
The present invention relates to a wavelength conversion element, a light source device, and a projector.
2. Related Art
In the past, there has been known a light source device irradiating a phosphor layer with excitation light emitted from a light source such as a laser to use the light emitted from the phosphor layer as illumination light. Further, in order to meet a need for increase in luminance in projectors, there has been proposed a projector provided with such a light source device (see, e.g., JP-A-2012-64484 (Document 1)).
The light source device described in Document 1 has a configuration in which phosphor ceramics is bonded to a light reflective substrate with a bonding material such as resin. The fluorescence generated by the phosphor is reflected by the light reflective substrate after passing through the bonding material, and is then emitted to the outside from the same surface as the surface the excitation enters.
However, every time the fluorescence is reflected by the light reflective substrate, a part of the fluorescence is converted into heat to thereby be absorbed. In other words, a loss of the fluorescence occurs. Therefore, there is a problem that the fluorescence generated by the phosphor cannot efficiently be used.
An advantage of some aspects of the invention is to provide a wavelength conversion element, a light source device, and a projector that are capable of efficiently use the fluorescence generated by the phosphor.
A wavelength conversion element according to an aspect of the invention includes a substrate, a reflecting section disposed on one surface side of the substrate, a wavelength conversion layer disposed on an opposite side of the reflecting section to the substrate, and emitting fluorescence in response to irradiation with excitation light, and a reflecting surface disposed between a surface of the wavelength conversion layer on an opposite side to the reflecting section and the reflecting section, and adapted to totally reflect light having been input at an angle no smaller than a critical angle out of the fluorescence.
According to the wavelength conversion element of this aspect of the invention, the light having entered the reflecting surface at an incident angle equal to or larger than a critical angle out of the fluorescence generated in the wavelength conversion layer is totally reflected by the reflecting surface. Therefore, compared to the case in which the reflecting surface is not provided, an amount of the fluorescence entering the reflecting section is small. In other words, the light loss due to the reflecting layer is low. Therefore, the use efficiency of the fluorescence generated by the wavelength conversion layer is high.
The wavelength conversion element according to the aspect of the invention may further include an intermediate layer disposed between the wavelength conversion layer and the reflecting section, and having a refractive index lower than a refractive index of the wavelength conversion layer is further included, wherein an interface between the wavelength conversion layer and the intermediate layer corresponds to the reflecting surface.
According to the aspect of the invention with this configuration, by disposing the intermediate layer having the refractive index lower than the refractive index of the wavelength conversion layer, the interface between the wavelength conversion layer and the intermediate layer can be used as the reflecting surface.
In the wavelength conversion element according to the aspect of the invention, the intermediate layer may have a thickness no smaller than a wavelength of the fluorescence.
According to this configuration, since a large proportion of the fluorescence is reflected by the reflecting surface, only little fluorescence enters the reflecting section. Thus, since an amount of the light absorbed by the reflecting section is small, the use efficiency of the fluorescence generated by the wavelength conversion layer is high.
In the wavelength conversion element according to the aspect of the invention, the thickness of the intermediate layer may be no smaller than 1 μm.
According to this configuration, since a large proportion of the fluorescence is reflected by the reflecting surface, only little fluorescence enters the reflecting section. Thus, since an amount of the light absorbed by the reflecting section is small, the use efficiency of the fluorescence generated by the wavelength conversion layer is high.
In the wavelength conversion element according to the aspect of the invention, the intermediate layer may be formed of an adhesive.
According to the aspect of the invention with this configuration, the bonding layer for bonding the wavelength conversion layer and the reflecting layer to each other can be used as the intermediate layer.
In the wavelength conversion element according to the aspect of the invention, thermal conductivity of the intermediate layer may be higher than thermal conductivity of the wavelength conversion layer.
According to the aspect of the invention with this configuration, the heat radiation performance of the intermediate layer is enhanced, and thus, the rise in temperature of the wavelength conversion layer can be suppressed.
In the wavelength conversion element according to the aspect of the invention, the reflecting surface may have an uneven shape.
According to this configuration, it is possible to provide a scattering property to the light reflected by the reflecting surface. Thus, it is possible to increase the proportion of the light, which can be emitted to an external space without being totally reflected by an interface between the light exit surface of the wavelength conversion element and the external space (air), out of the light having been reflected by the reflecting surface and then entered the interface.
In the aspect of the invention, the reflecting surface may be an interface between a void and a material located in a periphery of the void.
According to the aspect of the invention with this configuration, it is possible to make efficient use of the voids mixed or generated in the manufacturing process.
In the aspect of the invention, the wavelength conversion layer may include one of a plurality of crystal faults and a plurality of crystal grain boundaries, and the reflecting surface is formed of one of the crystal fault and the crystal grain boundary.
According to the aspect of the invention with this configuration, it is possible to make efficient use of the crystal faults and the crystal grain boundaries generated in the manufacturing process as the reflecting surface.
In the wavelength conversion element according to the aspect of the invention, the reflecting section may be a dichroic mirror adapted to transmit the excitation light and reflect the fluorescence.
According to this configuration, a transmissive wavelength conversion element can be provided. For example, in the case of adopting the configuration in which the excitation light is applied from another surface side on the opposite side of the one surface of the substrate, on which the dichroic mirror and the wavelength conversion layer are disposed, the excitation light is transmitted through the dichroic mirror, and then enters the wavelength conversion layer. It results that the light, which has proceeded toward the dichroic mirror, and then has been transmitted through the reflecting surface, out of the fluorescence emitted in response of the irradiation with the excitation light is reflected by the dichroic mirror.
In the wavelength conversion element according to the aspect of the invention, the wavelength conversion layer may include a plurality of materials different in refractive index from each other.
According to the aspect of the invention with this configuration, even in the case in which the wavelength conversion layer is formed of the plurality of materials different in refractive index from each other, by disposing the intermediate layer having the refractive index lower than the refractive indexes of these materials, it is possible to totally reflect the light at an incident angle equal to or larger than a critical angle out of the light emitted from the wavelength conversion layer with efficiency.
In the wavelength conversion element according to the aspect of the invention, the substrate may be rotatable around a rotational axis intersecting with the one surface of the substrate.
According to the aspect of the invention with this configuration, since the area of the wavelength conversion layer irradiated with the excitation light can temporally be moved, the temperature can be prevented from locally rising. Thus, the deterioration of the wavelength conversion layer can be prevented.
A light source device according to another aspect of the invention includes a light emitting element adapted to emit excitation light, and the wavelength conversion element described above.
According to this aspect of the invention, since the wavelength conversion element superior in use efficiency of the fluorescence is provided, the light source device capable of high luminance light emission can be provided.
In the light source device according to the aspect of the invention, when viewed from a direction parallel to a normal line of the one surface of the substrate, the reflecting surface may be disposed in a first area overlapping a second area of the wavelength conversion layer, the second area being irradiated with the excitation light.
According to the aspect of the invention with this configuration, the fluorescence, which has been excited by the excitation light and generated in the wavelength conversion layer, can totally be reflected by the total reflection surface with use efficiency.
A projector according to still another aspect of the invention includes the light source devices described above, a light modulation device adapted to modulate light from the light source device in accordance with image information, and a projection optical system adapted to project the modulated light from the light modulation device as a projection image.
According to this aspect of the invention, a projection image higher in quality can be projected.